[Paper Review] Capturing Nonlinear Electron Dynamics with Fully Characterised Attosecond X-ray Pulses

Lars Funke, Markus Ilchen+56 more|arXiv (Cornell University)|Aug 7, 2024

Advanced Electron Microscopy Techniques and ApplicationsBiochemistry, Genetics and Molecular Biology3 citations

TL;DR

This paper demonstrates the use of fully characterized attosecond X-ray pulses from a free-electron laser to capture nonlinear electron dynamics in real time, achieving sub-700-attosecond temporal resolution with up to 200 GW peak power. The method reveals transient electronic states via shot-to-shot pulse shape analysis, enabling element-specific probing of ultrafast charge redistribution and ionization dynamics in atoms like neon.

ABSTRACT

Attosecond X-ray pulses are the key to studying electron dynamics at their natural timescale in specifically targeted electronic states. They promise to build the conceptual bridge between physical and chemical photo-reaction processes. Free-electron lasers (FELs) have demonstrated their capability of generating intense attosecond X-ray pulses. The use of SASE-based FELs for time-resolving experiments and investigations of nonlinear X-ray absorption mechanisms, however, necessitates their full pulse-to-pulse characterisation which remains a cutting-edge challenge. We have characterised X-ray pulses with durations of down to 600 attoseconds and peak powers up to 200 GW at ~1 keV photon energy via angular streaking at the Small Quantum Systems instrument of the European XFEL in Germany. As a direct application, we present results of nonlinear X-ray--matter interaction via time-resolved electron spectroscopy on a transient system, observing single- and double-core-hole generation in neon atoms. Using the derived temporal information about each single X-ray pulse, we reveal an otherwise hidden peak-intensity dependence of the probability for formation of double-core vacancies in neon after primary K-shell ionisation. Our results advance the field of attosecond science with highly intense and fully characterised X-ray pulses to the state-specific investigation of electronic motion in non-stationary media.

Motivation & Objective

• To overcome limitations in attosecond X-ray science caused by the stochastic nature of SASE free-electron laser (XFEL) pulse generation.
• To enable precise, time-resolved observation of nonlinear electron dynamics in transient electronic states using fully characterized X-ray pulses.
• To develop a method that leverages shot-to-shot variations in X-ray pulse intensity and duration to extract otherwise hidden nonlinear dynamics.
• To establish a framework for element-specific, high-precision spectroscopy of ultrafast electronic processes in atoms and molecules.
• To map the connection between electronic rearrangement and subsequent nuclear dynamics in complex systems with attosecond resolution.

Proposed method

• Utilized isolated attosecond X-ray pulses generated at the European XFEL with durations of approximately 700 attoseconds and peak powers up to 200 GW.
• Performed time-resolved X-ray absorption and emission spectroscopy using neon as a target to probe core-level and valence electron dynamics.
• Employed shot-to-shot measurement of X-ray pulse intensity profiles to infer temporal structure and peak power variations.
• Applied a rate equation model to simulate Auger yields (SCH and DCH) based on calculated atomic cross sections and Auger decay lifetimes.
• Correlated experimental DCH (double core-hole) and SCH (single core-hole) signal variations with reconstructed pulse duration and fluence.
• Combined experimental data with theoretical modeling to validate the presence of nonlinear electron dynamics beyond linear response.

Experimental results

Research questions

• RQ1 Can isolated attosecond X-ray pulses with high peak power and precise temporal characterization resolve nonlinear electron dynamics in transient states?
• RQ2 How do shot-to-shot variations in X-ray pulse intensity and duration affect the detection of nonlinear processes like double core-hole creation?
• RQ3 To what extent can the temporal structure of SASE XFEL pulses be characterized to enable quantitative analysis of ultrafast electron dynamics?
• RQ4 What is the role of peak power and pulse duration in enhancing nonlinear ionization signals such as DCH production in neon?
• RQ5 Can the observed nonlinear dynamics be quantitatively explained by rate equation models based on atomic cross sections and decay lifetimes?

Key findings

• Isolated attosecond X-ray pulses with full temporal and spectral characterization were generated, achieving a pulse duration of about 700 as and peak power of up to 200 GW.
• Double core-hole (DCH) production signals showed strong dependence on the full width at half maximum (FWHM) of the reconstructed intensity profile, indicating sensitivity to peak power and pulse duration.
• Single core-hole (SCH) yields remained relatively constant across the FWHM range, confirming that DCH signals are specifically sensitive to nonlinear, high-intensity processes.
• The experimental DCH yield trends matched theoretical predictions from rate equation simulations based on atomic cross sections and Auger decay lifetimes.
• The method successfully revealed nonlinear electron dynamics in transient states that are otherwise hidden due to the stochastic nature of SASE XFEL pulses.
• The approach enables element-specific, time-resolved probing of electronic structure changes and charge redistribution on the attosecond scale, with potential for mapping electron-to-nuclear dynamics.